KC's 4-stroke 3-speed on GT2A-S gas tank frame

I have been itching to build a high end bike from the ground up on the cool Grubee GT2A frame designed specifically for motorizing from the in-frame 2.5L 2/3 gallon tank to the engine friendly frame geometry and I finally have my chance.

I have a customer that wants a 4-stroke 3-speed shifter like Big Red, but without the 'barnacle' gas tank.



I went through all the common options but as his budget grew so did the options until we got to GT2A frame which made for a ton more options.

Anyway, other than a base color theme and look I had him start with from the options of 3-speed wheel sets I have a big pallet to work with.

After an hour in my local bike shop with nothing but the bare painted frame and wheel set I got it on it's feet and rolled it out the bike shop door yesterday.



The is no easy way to explain how cool it is to have great little small business bike shop around corner that has actually grew to accommodate my business needs with motorized bicycle enthusiasm.

They have a work space for me to do things so I looked at tire options and finished the wheels and then moved on to picking the right fork, headset, handlebars and seat post by trying them on and looking at it.

But back to the build...
I had my local auto painting place paint the frame, chain guard, all the SBP 4-stroke shift kit parts and other misc parts to exactly match the rim color.

They did a nice job.
3 coats of primer, 2 coats of paint, and then 3 coats of sealer on everything.

Some parts specs:

Fork:
RST CAPA
Weight 1980g
Axle to Crown Length 460mm
Crown Race 30.0
Brake Type Cantilever, Disc, Linear Pull
Wheel Size 26"
Travel 80mm
Front Hub Spacing 100mm
Front Axle Type 9x1
Spring Coil
Wheel Mount Center Offset 0mm
Steerer Type Straight

KORE headset with 5 1/2" forward offset and 'just right' beach cruiser handlebars.

Tire Parts:
Standard inner liner.
Sunlite 6321 HD thorn resistant tube.
Thick 'special material' outer liner.
Kenda K1008 Flame 26x2.125" Cruiser Tires.

I really like Kenda tires and the cool flame tread design and not just because my wife's name is Kenda ;-}

Obviously more to come but I couldn't resist posting about this new build as it comes along from this point ;-}

*Note I made a topic here http://motorbicycling.com/showthread.php?t=52946 with pics and specs for a real close up look at what th4 GT2 frame is and what is really required to build any motorized bicycle with this frame as the base so please refer to it for frame details.

He's coming along nice.

It turns out the SickBikes 4-stroke Jackshaft kit motor mount base once all the muffler clamp parts are in place is over an 1" longer than the direct drive 4-stroke base the frame was designed for thus moving the engine position up to the point of the top bar interfering with function.

We spend the morning fabing some mount changes that should work and as the parts paint drys outside we went to work on getting the bike ready to just drop the mount and engine in and hook it up.

Pictures just don't compare to looking at it person and even riding it around as a pedal bike but I did what I could to get some pics from my alternative photo wall for this late time of day. (click them for big pics)











Needless to say we are chomping at the bit to get the engine in and ride it tomorrow ;-}

The customer wants to name it but a little picture of a flying wasp on each side of the tank would look cool I think, but I am already over budget and still can't get the engine in the way I want it.

GearNut said:

Subscribing with keen interest!
I am curious about the modifications you did to the mounting too.

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IMPORTANT NOTE!
Even the gasbike 4-stroke kit on their GT2A-S frame
Does NOT go in friendly!​

The stock Direct Drive goes on when made as short as possible so you can actually get to the choke lever but even all the way forward on that mount a regular 2-stroke gas petcock renders the air filter inaccessible without remove it AND the gas line.



I think my problem is trying to get the engine as low as possible like I have with the JS base which is even worse...



I am off to explore that right now but the design flaw in the frame was having the petcock fitting pointing straight down at the back of the top bar when they could have off-set to the right a bit.

scotto- said:

Just put a 90 degree elbow on the frame to angle the petcock out of the way.

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That was the first thing we tried this morning.
2 auto parts places, an Ace hardware, and a motorcycle parts place and nobody has anything that will fit that metric gas tank bung a 2/3 gallon Skyhawk petcock screws into, if you know of where I can get one please share.

Also a K&N or UNI air filter can give you more space and better performance. I know....you're already over budget, but air filters aren't real expensive

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Again, the closest I came to finding a different air cleaner that would fit the 142F was the SBP conversion one for their nice air cleaner but it is actually fatter.

Also be careful with that frame as it's made of very soft aluminum.

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Actually the material and welding seems top notch sturdy and strong to me.

13 hours without a break I have it almost figured out, even the throttle cable, and I took some pics of how I did it but that is for tomorrow as I'm beat and the re-paint of the mount parts are drying as is the epoxy job I did after cutting down that obnoxious choke lever until then.

The fix was to move the engine forward and to that it had to go up.

Here is the mod to the JS base I did.



To spread the 2-part SBP base wide enough the bottom plates middle slot for the set screw needs to be made longer in the back, long enough so the engines 2 front mounts only sit on the lower plate for this frame.

Note we tried to do it just a little shorter first and cut the top plates front slot all the way open, you won't need to do this is you go this wide.

When that it is done flip the base over and outline the bottom plates rear 2 slots on the top plate and make the slots that long.

We just drilled a slot size hole at the front end and then just worked the material out between the hole and slot but you don't have to do that either, all you need is rear motor mount bolt size holes in the top plate at the back.

In short with the motor mounted as far forward on the lower plate as it will go all you need to do is drill holes in the back of the top plate at the farthest forward point of the lower plates slots.

Since the front engine mount bolts won't help hold the the part base togeter and I don't trust just that single middle set to do much we put a couple of spot welds on the bottom to make sure the 2-part base stays together but if don't have a welder I would think a generous amount of JB Weld between the two plates and then clamping them together for 24 hours would do the job.

The result is the stock HS 142F on the GT2A-S frame with an SBP 4-stroke shift here.







This of course renders the engine and the base unmovable so go ahead and locktight your 4 engine bolts in well.

Today we tackle the tensioners on both sides as they will be needed to match the 4G's output to the actual jaskshaft on the left, and from the other JS side down to the BB sprocket.

The right side is easy, I put a hand made one using a gear from a 7-speed derailleur, a piece of metal, a bolts and some spacers and a spring on builds like this all the time...



There is a kit for this, maybe even SBP has one, but I like to fab things like this myself with what I always have 'laying around' so I don't need to buy parts ;-}

I'll try to detail that process as well so everyone that attempts a build like this in the future can get some advance tips.

scotto- said:

OK girls and boys, the frame of reference here (is) the Grubee GT2A

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Just for clarity this is NOT the GT2-A frame with the curved down tube and front 2-stroke mount pedestal in your reference pic, this is a GT2A-S frame with a straight downtube.
That has nothing to do with the quality of the metal used in both models, but structurally the S is superior.

Nashmoto is the guy who built this bike and he knows aluminum better than KC know's his wife......if Brian Nash tells me and also shows me how soft this aluminum frame is and is worried of it's strength, I just thought I'd pass on the info and fore warning of such.

If you don't get it, try riding it hard and do your double back flips and get back to us on how it's holding up......comprende?

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Good tip for someone that wants to build a high performance dirt bike or frigg'n stunt bike I guess, but I would take this frame over say a Schwinn Frontier for such things.
For example this POS Frontier frame that wouldn't take motorized street riding without breaking in a month.



(that story is for a new topic as it is not over)

Heck, I don't care if it's made out of hemp and horse piss, I rate the strength and quality of the GT2A-S between my low end minimum quality standards like a Macargi and a Giant frame.

The frame is not to blame for my current mount problem, I mounted the engine too far left.



(the engines top silver cap should be direct under the center of the top tab)

No biggie, I can put 4 little lateral slots in the base without taking it off the bike but it has re-inspired me to mock up my design for a base you attach this engine / 4G combo with a right site drive sprocket first, then you can just drop that unit onto the bike like a 2-stroke kit.

Sounds intriguing doesn't it ;-}

Not capping on the bike or build here....it looks great!

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Thanks bud, I am pretty pleased with how it is coming along myself ;-}

LR Jerry said:

A video of this bike being ridden would be nice. It be great if someone could film it from another bike or car.

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I'll get a video today if I can set the camera mode from still to video, it was dropped and the camera mode selector dial broke off and lost.

We took these two out together for first test rides yesterday.



The yellow 4-stroke 3-speed beat the 66cc direct drive in every way we could try with the 66cc on 25:1 break in gas and we didn't push hard and changed bikes often.

My wife is home today, I'll have her follow us and shoot some video when we test ride again.

I love the HS 142F and new 4G combo with a 3-speed, everything about it is just smooth and powerful enough for the job.

Thanks CARider,
Still not done, the chain guard is being a pain and we ran out of time and ambition today.

I had the auto body shop around the corner paint everything, I won't be doing that again, I'll go powder for everything next time.